Snake-eyes · Retinal anatomy and gene expression: a window to the origin of snakes

Retinal anatomy and gene expression: a window to the origin of snakes

Although clearly derived from a group of lizards, the origin of snakes is still subject to intense debates that currently revolve around two controversial hypotheses, a terrestrial and fossorial (burrowing) versus marine ancestry. Limited data on snake eyes and retinas have been used to support both hypotheses, mostly from older light microscope or gross morphology studies or from more recent molecular genetic perspectives, but thus far with very little input from immunohistochemistry, that integrates genetics and anatomy. Recent phylogenetic studies strongly support paraphyly of Scolecophidia (the group of “basal” highly fossorial blindsnakes and worm snakes), and this has been argued to provide evidence that the ancestral snake was ‘worm-like’ with reduced eyes. However, this has been disputed by molecular genetic studies, because alethinophidians (all other non-scolecophidian snakes) have retained some of the colour vision genes present in lizards. The lack of data about spatial patterns of retinal gene expression of some key scolecophidians precludes accurate reconstructions of the eye of the most recent common ancestor of all living snakes. It also precludes a more complete understanding of the functioning of the unusual retina of snakes. Thus, this proposal sought to populate this knowledge gap and solve these issues, by using immunohistochemistry and high-resolution imaging, to analyze the photoreceptors and opsins expressed in the retinas of key species from the two main lineages of Scolecophidia. By sampling five scolecophidian species from three families, including the two main branches of the group, I was able to conclude that in this highly fossorial and “basal” group, some species have retinas with two types of photoreceptors, rods and cones, and the three opsins found in alethinophidian snakes, SWS1, LWS, and RH1, thought to be missing in scolecophidians. Therefore, from the perspective of retinal anatomy and visual opsin expression, my data bring new lines of evidence to support the fossorial hypotheses for snake origins and to place scolecophidians as the most ancestral representatives of modern groups, shedding new light on the origin and ecological scenario of ancestral snakes. I also analyzed the retinal circuitry of caenophidian snakes, and concluded that the retinal architecture is highly divergent among diurnal and nocturnal species, with different types of interneurons and connectivity patterns with photoreceptors, which reveals astonishing adaptions of the visual system of snakes.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз